Subresolution grating for attenuated phase shifting mask fabrication

ABSTRACT

A subresolution grating composed of approximately circular contacts is fabricated around the border of the primary pattern of a photomask. As a result, resolution at the edges of the photomask pattern is improved when the pattern is printed on a wafer surface. In addition, the reduced leakage enables a more efficient use of the glass plate on which the photomask is fabricated as well as a more efficient use of the wafer surface as a result of being able to place patterns closer together.

This application is a continuation of U.S. Ser. No.: 09/004,183 filedJan. 8, 1998.

FIELD OF THE INVENTION

The present invention relates in general to methods and apparatus forphotolithography, and particularly to fabricating attenuating phaseshifting masks.

BACKGROUND OF THE INVENTION

There are three major methods of optically transferring a photomaskpattern to a wafer. These are contact printing, proximity printing, andprojection printing. In the fabrication of integrated circuitsprojection printing is used almost exclusively. In most conventionalsystems, each photomask is fabricated to include the primary pattern(the boundaries of the over-all circuit). Once a mask is complete, it isprinted many times on a wafer surface using a step-and-repeat processwhich is well-known in the art. The result is a pattern of dies coveringthe wafer surface.

In order to efficiently use the available wafer surface, the ability toaccurately and precisely transfer patterns onto the wafer surface iscritical. Thus, the equipment used to project patterns onto the wafersurface must provide, among other aspects, good resolution. The term`resolution` describes the ability of the optical system to distinguishclosely-spaced objects. The resolution of the optical lithographyprinting system is of major importance, since it is the main limitationof minimum device size. In modern projection printers the quality of theoptical elements is so high that their imaging characteristics arelimited by diffraction effects, and not by lens aberrations (diffractionlimited systems).

There are a variety of photomask used in conventional very large scaleintegration (VLSI) circuit fabrication processes. One widely used typeis the attenuated phase-shifting mask, which has the advantage of beinga two layer structure, simplifying the manufacturing process. Attenuatedphase-shifting masks, described in Burn J. Lin, "The AttenuatedPhase-Shift Mask", 43-47 Solid State Technology (January 1992), use aslightly transmissive absorber with a 180° phase-shift in the place ofthe opaque material in the mask pattern. Unlike many otherphase-shifting masks, attenuated phase-shifting masks can be used forany arbitrary mask pattern. An attenuated phase-shifting mask shifts thephase of dark areas but with an attenuated amplitude to preventproducing too much light in those areas. The negative amplitude providesthe desired improvement in image edge contrast, while the attenuationprevents the negative amplitude from becoming a problem by controllingthe intensity. Resolution of closely packed features is further improvedwhen using an attenuated phase-shifting mask incorporating a mask bias,because exposure times and diffractive effects can be reduced.

When a primary mask pattern is fabricated using attenuating phaseshifting material, the determination of where to locate adjacent diesmust provide for at least two considerations. One is that the mask platecannot extend to the edge of the die because leakage through theattenuated material at the edge of the primary pattern produces ashading effect, reducing the resolution of the pattern edge. The otherconcern is that, if adjacent dies are located too close together, theleakage at the edge of the mask pattern when printing one die willdetrimentally effect the neighboring die(s). In order to make efficientuse of the available wafer surface, there is a need to minimize thedetrimental optical effects occurring at the edges of the primary maskpattern. This would allow more precise definition of die boundaries andcloser placement of adjacent dies.

It is known in the art to employ diffraction gratings to improve maskfeature resolution by turning optical effects such as diffraction to anadvantage. Conventional diffraction gratings consist of rectangularfeatures arranged at equidistant intervals. It should be noted that, inintegrated circuit fabrication, a diffraction grating is not printed perse. Instead, the grating is employed to control optical effects such asdiffraction. The pattern of the grating produces destructiveinterference, thereby providing some amount of control over intensitypatterns on the wafer surface.

One example of a use for such patterns, often referred to as"subresolution gratings" or "zero electrical field gratings", is in themanufacture of attenuated phase-shifting masks, where the gratings areemployed to reduce the amount of light going through the attenuatedmaterial at the edge of the primary pattern. The subresolution gratingsused in conventional processing control optical effects by manipulatingboth the exposure parameters and the size and relative placement ofrectangular contacts. FIG. 1 shows an example of a portion of a maskincorporating a conventional subresolution grating. In the exampleshown, contacts 110 are separated by a space equal to the dimension ofthe contacts 110.

FIG. 2 is a graphic representation showing subresolution grating leakageversus contact size for the conventional subresolution grating ofFIG. 1. As shown in FIG. 2, the efficiency of the conventional gratingis somewhat improved when the contact size is reduced for all numericalapertures evaluated. As can be seen from graph 200, however, theconventional subresolution grating pattern experiences a minimumresidual intensity of at least 25% for even the smallest contact sizes.As a result, there is a point at which, using conventional methods,pattern resolution cannot be further improved. This creates a barrier tofurther efficiencies in printing mask patterns on a wafer. What isneeded is a method for fabricating a photomask with reduced energyleakage at the edges of the primary pattern.

SUMMARY OF THE INVENTION

The above mentioned problems with photomasks and other limitations areaddressed by the present invention which will be understood by readingand studying the following specification. According to one embodiment ofthe present invention a subresolution grating is incorporated in theborder demarcating a primary photomask pattern. In one embodiment thephotomask pattern incorporates phase-shifting techniques and is composedof attenuating material. In another embodiment the border is composed ofthe same material as the photomask. According to a further embodimentthe subresolution grating comprises approximately circular contacts.According to one implementation of the present invention the contactsare octagonal.

In another embodiment of the present invention a method for fabricatinga photomask with higher resolution is provided. The method comprises thesteps of preparing a glass plate, transferring a primary pattern ontothe glass plate, and fabricating a plurality of approximately circularcontacts around the edge of the primary pattern. According to oneembodiment the step of transferring a primary pattern comprisesselectively depositing attenuating material such that one or more of themask features operate as phase shifters. In another embodiment thefabrication step includes fabricating octagonal contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a portion of a conventional maskincorporating a subresolution grating.

FIG. 2 is a graphic representation showing subresolution grating leakageversus contact size for the conventional subresolution grating shown inFIG. 1.

FIG. 3 is a block diagram illustrating a portion of a photomaskincorporating a subresolution grating according to one embodiment of thepresent invention.

FIG. 4 is a graphic representation correlating subresolution leakage andcontact size in an environment incorporating the subresolution gratingof FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined by the appendedclaims.

It is well-known in the art to fabricate a border surrounding theprimary pattern of a photomask to aid in isolating individual patternsas they are printed on a wafer surface, and to serve as a buffer toreduce encroachment on previously printed patterns adjacent to locationwhere a pattern is being printed. When fabricating an attenuatedphase-shifting photomask, the designer has the option of bounding theprimary pattern with a deposition of either opaque chrome or attenuatingmaterial. The design parameters for either option have to take intoaccount optical effects (such as diffraction) which limit the attainableresolution of the pattern edges and may overwrite portions of adjacentpattern areas when the pattern is printed on a wafer surface. Asdescribed above, where the pattern edge consists of attenuatingmaterial, incorporating a subresolution pattern reduces the negativeoptical effects and improves pattern resolution.

Conventional subresolution gratings consist of regularly-spacedrectangular contacts. As discussed above, rectangular contacts arelimited in the amount of resolution they provide. The system of theinvention provides, among other features, a novel solution to thatlimitation. According to one embodiment of the present invention, thesubresolution grating incorporates contacts which are approximatelycircular. FIG. 3 is a block diagram illustrating a portion of aphotomask incorporating a subresolution grating according to oneembodiment of the present invention. In the example shown in FIG. 3,contacts 310 are octagonal in shape. Those skilled in the art willrecognize that the described embodiments are included for exemplarypurposes only and that other contact shapes which approximate a circlemay be incorporated in the diffraction grating without exceeding thespirit and scope of the present invention. According to the embodimentshown, the contacts are placed such that they are separated from eachother by a distance equal to the diameter of each contact 310. Thoseskilled in the art will recognize that other distances may be used,dependent upon the desired optical effects, without exceeding the spiritand scope of the present invention.

When the mask pattern is printed on the wafer surface, the diffractiongrating around the edge of the pattern creates a diffraction patternwhich reduces the zero-node diffraction to 0 and all other diffractionsto an angle large enough to miss the wafer surface all together. The netresult is improved resolution in the pattern edges, allowing the diepattern to extend closer to the edge of the mask plate and adjacentdaises to be located closer together.

FIG. 4 is a graphic representation correlating subresolution leakage andcontact size in an environment incorporating the embodiment of presentinvention shown in FIG. 3. As can be seen from the graph of FIG. 4, forcontact sizes of 0.11 microns or less there is virtually no residualintensity over a range of numerical apertures. The best performance isusing a contact size between 0.9 and 1.0 microns. By effectivelyreducing the leakage around the primary pattern to zero, the improvedsubresolution grating of the present invention enables high resolutionat the edges of the primary mask pattern. As a result, patternsincorporating the subresolution grating of the present invention may beprinted closer together on the wafer surface without impacting the maskquality or clarity.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A photomask produced by a methodcomprising:preparing a glass plate; transferring a primary pattern ontothe glass plate, the primary pattern including an edge circumscribingthe primary pattern; and fabricating a plurality of approximately roundcontacts in the edge of the primary pattern.
 2. The photomask of claim1, wherein fabricating a plurality of approximately round contactscomprises forming octagonal contacts.
 3. The photomask of claim 1,wherein transferring the primary pattern comprises selectivelydepositing attenuating material.
 4. A photomask produced by a methodcomprising:preparing a glass plate; selectively depositing attenuatingmaterial on the glass plate to form a primary pattern, the primarypattern including an edge circumscribing the primary pattern; andfabricating a plurality of octagonal contacts in the edge of the primarypattern.
 5. A method of transferring a pattern to a wafer,comprising:placing a photomask between an energy source and asemiconductor wafer, wherein the photomask includes a pattern such thata plurality of approximately round contacts are in an edgecircumscribing the pattern; and transferring an image of the pattern ofthe photomask on the semiconductor wafer through a controlledapplication of energy from the energy source.
 6. The method of claim 5,wherein the plurality of approximately round contacts include octagonalcontacts.
 7. The method of claim 5, wherein the pattern includesattenuating material.
 8. A subresolution grating, comprising:an borderoutlining a primary pattern of a photomask; and a plurality ofapproximately circular contacts in the border outlining the primarypattern.
 9. The subresolution grating of claim 8, wherein the pluralityof approximately circular contacts are octagonal.
 10. The subresolutiongrating of claim 8, wherein the plurality of approximately circularcontacts are subresolution in size.
 11. A subresolution grating,comprising:a border outlining a phase-shifting pattern, the border andthe phase-shifting pattern formed with attenuating material; and aplurality of approximately circular contacts in the border outlining thephase-shifting pattern.
 12. The subresolution grating of claim 11,wherein the plurality of approximately circular contacts are octagonal.13. A method of fabricating a photomask, comprising:transferring aprimary pattern onto a glass plate, the primary pattern including anedge circumscribing the primary pattern; and fabricating a plurality ofapproximately round contacts in the edge of the primary pattern.
 14. Themethod of claim 13, wherein fabricating a plurality of approximatelyround contacts comprises forming octagonal contacts.
 15. The method ofclaim 13, wherein transferring the primary pattern comprises selectivelydepositing attenuating material.
 16. A method of fabricating aphotomask, comprising:selectively depositing attenuating material on aglass plate to form a primary pattern, the primary pattern including anedge circumscribing the primary pattern; and fabricating a plurality ofoctagonal contacts in the edge of the primary pattern.
 17. A method offabricating a photomask, comprising:transferring a primary pattern ontoa glass plate, the primary pattern including an edge circumscribing theprimary pattern; and fabricating a plurality of approximately roundcontacts in the edge of the primary pattern, such that adjacent contactsof the plurality of approximately round contacts are separated by adistance approximately equal to a diameter of one of the plurality ofapproximately round contacts.
 18. The method of claim 17, whereinfabricating the plurality of approximately round contacts includesforming octagonal contacts.
 19. The method of claim 17, whereintransferring the primary pattern comprises selectively depositingattenuating material.
 20. A method of fabricating a photomask,comprising:transferring a primary pattern onto a glass plate, theprimary pattern including an edge circumscribing the primary pattern;and fabricating a plurality of octagonal contacts in the edge of theprimary pattern, such that adjacent contacts of the plurality ofoctagonal contacts are separated by a distance approximately equal to awidth of one of the plurality of octagonal contacts.
 21. The method ofclaim 20, wherein transferring the primary pattern comprises selectivelydepositing attenuating material.
 22. A photomask, comprising:a primarypattern; a border outlining the primary pattern; and a plurality ofapproximately circular contacts in the border outlining the primarypattern such that adjacent contacts of the plurality of approximatelycircular contacts are separated by a distance approximately equal to adiameter of one of the plurality of approximately circular contacts. 23.The photomask of claim 22, wherein the primary pattern and the borderare composed of attenuating phase-shifting material.
 24. The photomaskof claim 23, wherein the plurality of approximately circular contactsare octagonal.
 25. The photomask of claim 23, wherein the plurality ofapproximately circular contacts are subresolution in size.
 26. Aphotomask, comprising:a phase-shifting pattern formed with attenuatingmaterial, including a border circumscribing the phase-shifting pattern;a plurality of approximately circular contacts formed in the border,such that adjacent contacts of the plurality of approximately circularcontacts are separated by a distance approximately equal to a diameterof one of the plurality of approximately circular contacts.
 27. Thephotomask of claim 26, wherein the plurality of approximately circularcontacts are octagonal.